Method and apparatus for video coding

ABSTRACT

According to an aspect of the disclosure, processing circuitry decodes a constrain flag from a coded video bitstream. The constrain flag is indicative of an exclusion of decoder-side motion vector derivation (DMVD) for reference sample reconstruction. Further, the processing circuitry decodes prediction information of a current block from the coded video bitstream. The prediction information is indicative of an intra prediction mode. Then, the processing circuitry determines, in a same picture as the current block, reference samples for a sample in the current block based on the intra prediction mode and based on the exclusion of the DMVD, and reconstructs the sample of the current block according to the reference samples.

INCORPORATION BY REFERENCE

This present disclosure claims the benefit of priority to U.S.Provisional Application No. 62/731,786, “FILTERS AND INTRA PREDICTIONCONSTRAINTS IN DECODER SIDE MOTION VECTOR DERIVATION AND REFINEMENT”filed on Sep. 14, 2018, which is incorporated by reference herein in itsentirety.

TECHNICAL FIELD

The present disclosure describes embodiments generally related to videocoding.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent the work is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

Video coding and decoding can be performed using inter-pictureprediction with motion compensation. Uncompressed digital video caninclude a series of pictures, each picture having a spatial dimensionof, for example, 1920×1080 luminance samples and associated chrominancesamples. The series of pictures can have a fixed or variable picturerate (informally also known as frame rate), of, for example 60 picturesper second or 60 Hz. Uncompressed video has significant bitraterequirements. For example, 1080p60 4:2:0 video at 8 bit per sample(1920×1080 luminance sample resolution at 60 Hz frame rate) requiresclose to 1.5 Gbit/s bandwidth. An hour of such video requires more than600 GBytes of storage space.

One purpose of video coding and decoding can be the reduction ofredundancy in the input video signal, through compression. Compressioncan help reduce the aforementioned bandwidth or storage spacerequirements, in some cases by two orders of magnitude or more. Bothlossless and lossy compression, as well as a combination thereof can beemployed. Lossless compression refers to techniques where an exact copyof the original signal can be reconstructed from the compressed originalsignal. When using lossy compression, the reconstructed signal may notbe identical to the original signal, but the distortion between originaland reconstructed signals is small enough to make the reconstructedsignal useful for the intended application. In the case of video, lossycompression is widely employed. The amount of distortion tolerateddepends on the application; for example, users of certain consumerstreaming applications may tolerate higher distortion than users oftelevision distribution applications. The compression ratio achievablecan reflect that:higher allowable/tolerable distortion can yield highercompression ratios.

Motion compensation can be a lossy compression technique and can relateto techniques where a block of sample data from a previouslyreconstructed picture or part thereof (reference picture), after beingspatially shifted in a direction indicated by a motion vector (MVhenceforth), is used for the prediction of a newly reconstructed pictureor picture part. In some cases, the reference picture can be the same asthe picture currently under reconstruction. MVs can have two dimensionsX and Y, or three dimensions, the third being an indication of thereference picture in use (the latter, indirectly, can be a timedimension).

In some video compression techniques, an MV applicable to a certain areaof sample data can be predicted from other MVs, for example from thoserelated to another area of sample data spatially adjacent to the areaunder reconstruction, and preceding that MV in decoding order. Doing socan substantially reduce the amount of data required for coding the MV,thereby removing redundancy and increasing compression. MV predictioncan work effectively, for example, because when coding an input videosignal derived from a camera (known as natural video) there is astatistical likelihood that areas larger than the area to which a singleMV is applicable move in a similar direction and, therefore, can in somecases be predicted using a similar motion vector derived from MVs ofneighboring area. That results in the MV found for a given area to besimilar or the same as the MV predicted from the surrounding MVs, andthat in turn can be represented, after entropy coding, in a smallernumber of bits than what would be used if coding the MV directly. Insome cases, MV prediction can be an example of lossless compression of asignal (namely: the MVs) derived from the original signal (namely: thesample stream). In other cases, MV prediction itself can be lossy, forexample because of rounding errors when calculating a predictor fromseveral surrounding MVs.

Various MV prediction mechanisms are described in H.265/HEVC (ITU-T Rec.H.265, “High Efficiency Video Coding”, December 2016). Out of the manyMV prediction mechanisms that H.265 offers, described here is atechnique henceforth referred to as “spatial merge”.

SUMMARY

Aspects of the disclosure provide methods and apparatuses for videoencoding/decoding. In some examples, an apparatus for video decodingincludes receiving circuitry and processing circuitry.

According to an aspect of the disclosure, the processing circuitrydecodes a constrain flag from a coded video bitstream. The constrainflag is indicative of an exclusion of decoder-side motion vectorderivation (DMVD) for reference sample reconstruction. Further, theprocessing circuitry decodes prediction information of a current blockfrom the coded video bitstream. The prediction information is indicativeof an intra prediction mode. Then, the processing circuitry determines,in a same picture as the current block, reference samples for a samplein the current block based on the intra prediction mode and based on theexclusion of the DMVD, and reconstructs the sample of the current blockaccording to the reference samples.

In some embodiments, the processing circuitry determines the referencesamples according to the intra prediction mode with the referencesamples being reconstructed without using the DMVD.

In an embodiment, the constrain flag has a first potential valueindicative of a permit of reference sample reconstruction using intraprediction, has a second potential value indicative of a permit ofreference sample reconstruction using intra prediction and interprediction, and has a third potential value indicative of the exclusionof the DMVD.

In another embodiment, the processing circuitry decodes a firstconstrain flag indicative of a permit of reference sample reconstructionusing intra prediction and inter prediction, and decodes the constrainflag that is a second constrain flag indicative of the exclusion of theDMVD for the reference sample construction.

In another embodiment, the processing circuitry decodes a firstconstrain flag indicative of a permit of reference sample reconstructionusing intra prediction, and decodes the constrain flag that is a secondconstrain flag indicative of the exclusion of the DMVD for the referencesample construction.

In another embodiment, the processing circuitry decodes a firstconstrain flag indicative of a permit of reference sample reconstructionusing intra prediction, and infers, based on the first constrain flag,the exclusion of the DMVD for the reference sample construction.

In another embodiment, the processing circuitry determines the referencesamples in a DMVD non-latency region according to the intra predictionmode with the reference samples available for the reconstruction of thecurrent block without delay.

According to another aspect of the disclosure, the processing circuitrydecodes prediction information for a current block in a current picturefrom a coded video bitstream. The prediction information is indicativeof an inter prediction mode that includes a motion vector refining stepand a final motion compensation step. Then, the processing circuitryuses a first interpolation filter during the motion vector refining stepto determine a first refined motion vector that is indicative of a firstrefined reference block in a first reference frame from an initialmotion vector. Further, the processing circuitry uses a secondinterpolation filter that is different from the first interpolationfilter during the final motion compensation step to reconstruct thecurrent block according to the first refined motion vector.

In some embodiments, the processing circuitry uses the firstinterpolation filter during the motion vector refining step to determinethe first refined motion vector that is indicative of a first refinedreference block in a first reference frame and a second refined motionvector that is indicative of a second refined reference block in asecond reference frame. Further, the processing circuitry uses thesecond interpolation filter during the final motion compensation step toreconstruct the current block according to the first refined motionvector and the second refined motion vector.

In some examples, the first interpolation filter has equal or fewernumber of taps than the second interpolation filter.

In some embodiments, the inter prediction mode is decoder-side motionvector derivation (DMVD) mode, and the first interpolation filter andthe second interpolation filter are different from a third interpolationfilter that is used in a non-DMVD mode. In some examples, the secondinterpolation filter has equal or fewer number of taps than the thirdinterpolation filter. For example, a subtraction of a third number oftaps of the third interpolation filter and a second number of taps ofthe second interpolation filter is larger than two times of the searchrange in pixel.

According to an aspect of the disclosure, the processing circuitryselects the second interpolation filter based on a change in an integerportion of the first refined motion vector. In an example, when thefirst refined motion vector and the initial motion vector have a sameinteger portion, the processing circuitry uses the third interpolationfilter to replace the second interpolation filter during the finalmotion compensation step to reconstruct the current block according tothe first refined motion vector. In another example, when the firstrefined motion vector and the initial motion vector have differentinteger portions, the processing circuitry uses the first interpolationfilter to replace the second interpolation filter during the finalmotion compensation step to reconstruct the current block according tothe first refined motion vector.

In some examples, the first interpolation filter and the secondinterpolation filter have lower cutoff frequency than the thirdinterpolation filter.

Aspects of the disclosure also provide a non-transitorycomputer-readable medium storing instructions which when executed by acomputer for video decoding cause the computer to perform the methodsfor video decoding.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, the nature, and various advantages of the disclosedsubject matter will be more apparent from the following detaileddescription and the accompanying drawings in which:

FIG. 1 is a schematic illustration of a simplified block diagram of acommunication system (100) in accordance with an embodiment.

FIG. 2 is a schematic illustration of a simplified block diagram of acommunication system (200) in accordance with an embodiment.

FIG. 3 is a schematic illustration of a simplified block diagram of adecoder in accordance with an embodiment.

FIG. 4 is a schematic illustration of a simplified block diagram of anencoder in accordance with an embodiment.

FIG. 5 shows a block diagram of an encoder in accordance with anotherembodiment.

FIG. 6 shows a block diagram of a decoder in accordance with anotherembodiment.

FIG. 7 shows a current block (701) and spatial merge candidates in someexamples.

FIG. 8 shows an example of DMVR that is based on bilateral templatematching.

FIG. 9 shows a diagram of search spaces according to an embodiment ofthe disclosure.

FIG. 10 shows a diagram for half-sample precision search in an example.

FIG. 11 shows an example of intra prediction according to an embodimentof the disclosure.

FIG. 12 shows a diagram illustrating DMVR latency region and DMVRnon-latency region according to an embodiment of the disclosure.

FIG. 13 shows a flow chart outlining a process example according to anembodiment of the disclosure;

FIG. 14 shows a flow chart outlining another process example accordingto an embodiment of the disclosure.

FIG. 15 is a schematic illustration of a computer system in accordancewith an embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 illustrates a simplified block diagram of a communication system(100) according to an embodiment of the present disclosure. Thecommunication system (100) includes a plurality of terminal devices thatcan communicate with each other, via, for example, a network (150). Forexample, the communication system (100) includes a first pair ofterminal devices (110) and (120) interconnected via the network (150).In the FIG. 1 example, the first pair of terminal devices (110) and(120) performs unidirectional transmission of data. For example, theterminal device (110) may code video data (e.g., a stream of videopictures that are captured by the terminal device (110)) fortransmission to the other terminal device (120) via the network (150).The encoded video data can be transmitted in the form of one or morecoded video bitstreams. The terminal device (120) may receive the codedvideo data from the network (150), decode the coded video data torecover the video pictures and display video pictures according to therecovered video data. Unidirectional data transmission may be common inmedia serving applications and the like.

In another example, the communication system (100) includes a secondpair of terminal devices (130) and (140) that performs bidirectionaltransmission of coded video data that may occur, for example, duringvideoconferencing. For bidirectional transmission of data, in anexample, each terminal device of the terminal devices (130) and (140)may code video data (e.g., a stream of video pictures that are capturedby the terminal device) for transmission to the other terminal device ofthe terminal devices (130) and (140) via the network (150). Eachterminal device of the terminal devices (130) and (140) also may receivethe coded video data transmitted by the other terminal device of theterminal devices (130) and (140), and may decode the coded video data torecover the video pictures and may display video pictures at anaccessible display device according to the recovered video data.

In the FIG. 1 example, the terminal devices (110), (120), (130) and(140) may be illustrated as servers, personal computers and smart phonesbut the principles of the present disclosure may be not so limited.Embodiments of the present disclosure find application with laptopcomputers, tablet computers, media players and/or dedicated videoconferencing equipment. The network (150) represents any number ofnetworks that convey coded video data among the terminal devices (110),(120), (130) and (140), including for example wireline (wired) and/orwireless communication networks. The communication network (150) mayexchange data in circuit-switched and/or packet-switched channels.Representative networks include telecommunications networks, local areanetworks, wide area networks and/or the Internet. For the purposes ofthe present discussion, the architecture and topology of the network(150) may be immaterial to the operation of the present disclosureunless explained herein below.

FIG. 2 illustrates, as an example for an application for the disclosedsubject matter, the placement of a video encoder and a video decoder ina streaming environment. The disclosed subject matter can be equallyapplicable to other video enabled applications, including, for example,video conferencing, digital TV, storing of compressed video on digitalmedia including CD, DVD, memory stick and the like, and so on.

A streaming system may include a capture subsystem (213), that caninclude a video source (201), for example a digital camera, creating forexample a stream of video pictures (202) that are uncompressed. In anexample, the stream of video pictures (202) includes samples that aretaken by the digital camera. The stream of video pictures (202),depicted as a bold line to emphasize a high data volume when compared toencoded video data (204) (or coded video bitstreams), can be processedby an electronic device (220) that includes a video encoder (203)coupled to the video source (201). The video encoder (203) can includehardware, software, or a combination thereof to enable or implementaspects of the disclosed subject matter as described in more detailbelow. The encoded video data (204) (or encoded video bitstream (204)),depicted as a thin line to emphasize the lower data volume when comparedto the stream of video pictures (202), can be stored on a streamingserver (205) for future use. One or more streaming client subsystems,such as client subsystems (206) and (208) in FIG. 2 can access thestreaming server (205) to retrieve copies (207) and (209) of the encodedvideo data (204). A client subsystem (206) can include a video decoder(210), for example, in an electronic device (230). The video decoder(210) decodes the incoming copy (207) of the encoded video data andcreates an outgoing stream of video pictures (211) that can be renderedon a display (212) (e.g., display screen) or other rendering device (notdepicted). In some streaming systems, the encoded video data (204),(207), and (209) (e.g., video bitstreams) can be encoded according tocertain video coding/compression standards. Examples of those standardsinclude ITU-T Recommendation H.265. In an example, a video codingstandard under development is informally known as Versatile Video Coding(VVC). The disclosed subject matter may be used in the context of VVC.

It is noted that the electronic devices (220) and (230) can includeother components (not shown). For example, the electronic device (220)can include a video decoder (not shown) and the electronic device (230)can include a video encoder (not shown) as well.

FIG. 3 shows a block diagram of a video decoder (310) according to anembodiment of the present disclosure. The video decoder (310) can beincluded in an electronic device (330). The electronic device (330) caninclude a receiver (331) (e.g., receiving circuitry). The video decoder(310) can be used in the place of the video decoder (210) in the FIG. 2example.

The receiver (331) may receive one or more coded video sequences to bedecoded by the video decoder (310); in the same or another embodiment,one coded video sequence at a time, where the decoding of each codedvideo sequence is independent from other coded video sequences. Thecoded video sequence may be received from a channel (301), which may bea hardware/software link to a storage device which stores the encodedvideo data. The receiver (331) may receive the encoded video data withother data, for example, coded audio data and/or ancillary data streams,that may be forwarded to their respective using entities (not depicted).The receiver (331) may separate the coded video sequence from the otherdata. To combat network jitter, a buffer memory (315) may be coupled inbetween the receiver (331) and an entropy decoder/parser (320) (“parser(320)” henceforth). In certain applications, the buffer memory (315) ispart of the video decoder (310). In others, it can be outside of thevideo decoder (310) (not depicted). In still others, there can be abuffer memory (not depicted) outside of the video decoder (310), forexample to combat network jitter, and in addition another buffer memory(315) inside the video decoder (310), for example to handle playouttiming. When the receiver (331) is receiving data from a store/forwarddevice of sufficient bandwidth and controllability, or from anisosynchronous network, the buffer memory (315) may not be needed, orcan be small. For use on best effort packet networks such as theInternet, the buffer memory (315) may be required, can be comparativelylarge and can be advantageously of adaptive size, and may at leastpartially be implemented in an operating system or similar elements (notdepicted) outside of the video decoder (310).

The video decoder (310) may include the parser (320) to reconstructsymbols (321) from the coded video sequence. Categories of those symbolsinclude information used to manage operation of the video decoder (310),and potentially information to control a rendering device such as arender device (312) (e.g., a display screen) that is not an integralpart of the electronic device (330) but can be coupled to the electronicdevice (330), as was shown in FIG. 3. The control information for therendering device(s) may be in the form of Supplemental EnhancementInformation (SEI messages) or Video Usability Information (VUI)parameter set fragments (not depicted). The parser (320) mayparse/entropy-decode the coded video sequence that is received. Thecoding of the coded video sequence can be in accordance with a videocoding technology or standard, and can follow various principles,including variable length coding, Huffman coding, arithmetic coding withor without context sensitivity, and so forth. The parser (320) mayextract from the coded video sequence, a set of subgroup parameters forat least one of the subgroups of pixels in the video decoder, based uponat least one parameter corresponding to the group. Subgroups can includeGroups of Pictures (GOPs), pictures, tiles, slices, macroblocks, CodingUnits (CUs), blocks, Transform Units (TUs), Prediction Units (PUs) andso forth. The parser (320) may also extract from the coded videosequence information such as transform coefficients, quantizer parametervalues, motion vectors, and so forth.

The parser (320) may perform an entropy decoding/parsing operation onthe video sequence received from the buffer memory (315), so as tocreate symbols (321).

Reconstruction of the symbols (321) can involve multiple different unitsdepending on the type of the coded video picture or parts thereof (suchas: inter and intra picture, inter and intra block), and other factors.Which units are involved, and how, can be controlled by the subgroupcontrol information that was parsed from the coded video sequence by theparser (320). The flow of such subgroup control information between theparser (320) and the multiple units below is not depicted for clarity.

Beyond the functional blocks already mentioned, the video decoder (310)can be conceptually subdivided into a number of functional units asdescribed below. In a practical implementation operating undercommercial constraints, many of these units interact closely with eachother and can, at least partly, be integrated into each other. However,for the purpose of describing the disclosed subject matter, theconceptual subdivision into the functional units below is appropriate.

A first unit is the scaler/inverse transform unit (351). Thescaler/inverse transform unit (351) receives a quantized transformcoefficient as well as control information, including which transform touse, block size, quantization factor, quantization scaling matrices,etc. as symbol(s) (321) from the parser (320). The scaler/inversetransform unit (351) can output blocks comprising sample values, thatcan be input into aggregator (355).

In some cases, the output samples of the scaler/inverse transform (351)can pertain to an intra coded block; that is: a block that is not usingpredictive information from previously reconstructed pictures, but canuse predictive information from previously reconstructed parts of thecurrent picture. Such predictive information can be provided by an intrapicture prediction unit (352). In some cases, the intra pictureprediction unit (352) generates a block of the same size and shape ofthe block under reconstruction, using surrounding already reconstructedinformation fetched from the current picture buffer (358). The currentpicture buffer (358) buffers, for example, partly reconstructed currentpicture and/or fully reconstructed current picture. The aggregator(355), in some cases, adds, on a per sample basis, the predictioninformation the intra prediction unit (352) has generated to the outputsample information as provided by the scaler/inverse transform unit(351).

In other cases, the output samples of the scaler/inverse transform unit(351) can pertain to an inter coded, and potentially motion compensatedblock. In such a case, a motion compensation prediction unit (353) canaccess reference picture memory (357) to fetch samples used forprediction. After motion compensating the fetched samples in accordancewith the symbols (321) pertaining to the block, these samples can beadded by the aggregator (355) to the output of the scaler/inversetransform unit (351) (in this case called the residual samples orresidual signal) so as to generate output sample information. Theaddresses within the reference picture memory (357) from where themotion compensation prediction unit (353) fetches prediction samples canbe controlled by motion vectors, available to the motion compensationprediction unit (353) in the form of symbols (321) that can have, forexample X, Y, and reference picture components. Motion compensation alsocan include interpolation of sample values as fetched from the referencepicture memory (357) when sub-sample exact motion vectors are in use,motion vector prediction mechanisms, and so forth.

The output samples of the aggregator (355) can be subject to variousloop filtering techniques in the loop filter unit (356). Videocompression technologies can include in-loop filter technologies thatare controlled by parameters included in the coded video sequence (alsoreferred to as coded video bitstream) and made available to the loopfilter unit (356) as symbols (321) from the parser (320), but can alsobe responsive to meta-information obtained during the decoding ofprevious (in decoding order) parts of the coded picture or coded videosequence, as well as responsive to previously reconstructed andloop-filtered sample values.

The output of the loop filter unit (356) can be a sample stream that canbe output to the render device (312) as well as stored in the referencepicture memory (357) for use in future inter-picture prediction.

Certain coded pictures, once fully reconstructed, can be used asreference pictures for future prediction. For example, once a codedpicture corresponding to a current picture is fully reconstructed andthe coded picture has been identified as a reference picture (by, forexample, the parser (320)), the current picture buffer (358) can becomea part of the reference picture memory (357), and a fresh currentpicture buffer can be reallocated before commencing the reconstructionof the following coded picture.

The video decoder (310) may perform decoding operations according to apredetermined video compression technology in a standard, such as ITU-TRec. H.265. The coded video sequence may conform to a syntax specifiedby the video compression technology or standard being used, in the sensethat the coded video sequence adheres to both the syntax of the videocompression technology or standard and the profiles as documented in thevideo compression technology or standard. Specifically, a profile canselect certain tools as the only tools available for use under thatprofile from all the tools available in the video compression technologyor standard. Also necessary for compliance can be that the complexity ofthe coded video sequence is within bounds as defined by the level of thevideo compression technology or standard. In some cases, levels restrictthe maximum picture size, maximum frame rate, maximum reconstructionsample rate (measured in, for example megasamples per second), maximumreference picture size, and so on. Limits set by levels can, in somecases, be further restricted through Hypothetical Reference Decoder(HRD) specifications and metadata for HRD buffer management signaled inthe coded video sequence.

In an embodiment, the receiver (331) may receive additional (redundant)data with the encoded video. The additional data may be included as partof the coded video sequence(s). The additional data may be used by thevideo decoder (310) to properly decode the data and/or to moreaccurately reconstruct the original video data. Additional data can bein the form of, for example, temporal, spatial, or signal noise ratio(SNR) enhancement layers, redundant slices, redundant pictures, forwarderror correction codes, and so on.

FIG. 4 shows a block diagram of a video encoder (403) according to anembodiment of the present disclosure. The video encoder (403) isincluded in an electronic device (420). The electronic device (420)includes a transmitter (440) (e.g., transmitting circuitry). The videoencoder (403) can be used in the place of the video encoder (203) in theFIG. 2 example.

The video encoder (403) may receive video samples from a video source(401) (that is not part of the electronic device (420) in the FIG. 4example) that may capture video image(s) to be coded by the videoencoder (403). In another example, the video source (401) is a part ofthe electronic device (420).

The video source (401) may provide the source video sequence to be codedby the video encoder (403) in the form of a digital video sample streamthat can be of any suitable bit depth (for example: 8 bit, 10 bit, 12bit, . . . ), any colorspace (for example, BT.601 Y CrCB, RGB, . . . ),and any suitable sampling structure (for example Y CrCb 4:2:0, Y CrCb4:4:4). In a media serving system, the video source (401) may be astorage device storing previously prepared video. In a videoconferencingsystem, the video source (401) may be a camera that captures local imageinformation as a video sequence. Video data may be provided as aplurality of individual pictures that impart motion when viewed insequence. The pictures themselves may be organized as a spatial array ofpixels, wherein each pixel can comprise one or more samples depending onthe sampling structure, color space, etc. in use. A person skilled inthe art can readily understand the relationship between pixels andsamples. The description below focuses on samples.

According to an embodiment, the video encoder (403) may code andcompress the pictures of the source video sequence into a coded videosequence (443) in real time or under any other time constraints asrequired by the application. Enforcing appropriate coding speed is onefunction of a controller (450). In some embodiments, the controller(450) controls other functional units as described below and isfunctionally coupled to the other functional units. The coupling is notdepicted for clarity. Parameters set by the controller (450) can includerate control related parameters (picture skip, quantizer, lambda valueof rate-distortion optimization techniques, . . . ), picture size, groupof pictures (GOP) layout, maximum motion vector search range, and soforth. The controller (450) can be configured to have other suitablefunctions that pertain to the video encoder (403) optimized for acertain system design.

In some embodiments, the video encoder (403) is configured to operate ina coding loop. As an oversimplified description, in an example, thecoding loop can include a source coder (430) (e.g., responsible forcreating symbols, such as a symbol stream, based on an input picture tobe coded, and a reference picture(s)), and a (local) decoder (433)embedded in the video encoder (403). The decoder (433) reconstructs thesymbols to create the sample data in a similar manner as a (remote)decoder also would create (as any compression between symbols and codedvideo bitstream is lossless in the video compression technologiesconsidered in the disclosed subject matter). The reconstructed samplestream (sample data) is input to the reference picture memory (434). Asthe decoding of a symbol stream leads to bit-exact results independentof decoder location (local or remote), the content in the referencepicture memory (434) is also bit exact between the local encoder andremote encoder. In other words, the prediction part of an encoder “sees”as reference picture samples exactly the same sample values as a decoderwould “see” when using prediction during decoding. This fundamentalprinciple of reference picture synchronicity (and resulting drift, ifsynchronicity cannot be maintained, for example because of channelerrors) is used in some related arts as well.

The operation of the “local” decoder (433) can be the same as of a“remote” decoder, such as the video decoder (310), which has alreadybeen described in detail above in conjunction with FIG. 3. Brieflyreferring also to FIG. 3, however, as symbols are available andencoding/decoding of symbols to a coded video sequence by an entropycoder (445) and the parser (320) can be lossless, the entropy decodingparts of the video decoder (310), including the buffer memory (315), andparser (320) may not be fully implemented in the local decoder (433).

An observation that can be made at this point is that any decodertechnology except the parsing/entropy decoding that is present in adecoder also necessarily needs to be present, in substantially identicalfunctional form, in a corresponding encoder. For this reason, thedisclosed subject matter focuses on decoder operation. The descriptionof encoder technologies can be abbreviated as they are the inverse ofthe comprehensively described decoder technologies. Only in certainareas a more detail description is required and provided below.

During operation, in some examples, the source coder (430) may performmotion compensated predictive coding, which codes an input picturepredictively with reference to one or more previously-coded picture fromthe video sequence that were designated as “reference pictures”. In thismanner, the coding engine (432) codes differences between pixel blocksof an input picture and pixel blocks of reference picture(s) that may beselected as prediction reference(s) to the input picture.

The local video decoder (433) may decode coded video data of picturesthat may be designated as reference pictures, based on symbols createdby the source coder (430). Operations of the coding engine (432) mayadvantageously be lossy processes. When the coded video data may bedecoded at a video decoder (not shown in FIG. 4), the reconstructedvideo sequence typically may be a replica of the source video sequencewith some errors. The local video decoder (433) replicates decodingprocesses that may be performed by the video decoder on referencepictures and may cause reconstructed reference pictures to be stored inthe reference picture cache (434). In this manner, the video encoder(403) may store copies of reconstructed reference pictures locally thathave common content as the reconstructed reference pictures that will beobtained by a far-end video decoder (absent transmission errors).

The predictor (435) may perform prediction searches for the codingengine (432). That is, for a new picture to be coded, the predictor(435) may search the reference picture memory (434) for sample data (ascandidate reference pixel blocks) or certain metadata such as referencepicture motion vectors, block shapes, and so on, that may serve as anappropriate prediction reference for the new pictures. The predictor(435) may operate on a sample block-by-pixel block basis to findappropriate prediction references. In some cases, as determined bysearch results obtained by the predictor (435), an input picture mayhave prediction references drawn from multiple reference pictures storedin the reference picture memory (434).

The controller (450) may manage coding operations of the source coder(430), including, for example, setting of parameters and subgroupparameters used for encoding the video data.

Output of all aforementioned functional units may be subjected toentropy coding in the entropy coder (445). The entropy coder (445)translates the symbols as generated by the various functional units intoa coded video sequence, by lossless compressing the symbols according totechnologies such as Huffman coding, variable length coding, arithmeticcoding, and so forth.

The transmitter (440) may buffer the coded video sequence(s) as createdby the entropy coder (445) to prepare for transmission via acommunication channel (460), which may be a hardware/software link to astorage device which would store the encoded video data. The transmitter(440) may merge coded video data from the video coder (403) with otherdata to be transmitted, for example, coded audio data and/or ancillarydata streams (sources not shown).

The controller (450) may manage operation of the video encoder (403).During coding, the controller (450) may assign to each coded picture acertain coded picture type, which may affect the coding techniques thatmay be applied to the respective picture. For example, pictures oftenmay be assigned as one of the following picture types:

An Intra Picture (I picture) may be one that may be coded and decodedwithout using any other picture in the sequence as a source ofprediction. Some video codecs allow for different types of intrapictures, including, for example Independent Decoder Refresh (“IDR”)Pictures. A person skilled in the art is aware of those variants of Ipictures and their respective applications and features.

A predictive picture (P picture) may be one that may be coded anddecoded using intra prediction or inter prediction using at most onemotion vector and reference index to predict the sample values of eachblock.

A bi-directionally predictive picture (B Picture) may be one that may becoded and decoded using intra prediction or inter prediction using atmost two motion vectors and reference indices to predict the samplevalues of each block. Similarly, multiple-predictive pictures can usemore than two reference pictures and associated metadata for thereconstruction of a single block.

Source pictures commonly may be subdivided spatially into a plurality ofsample blocks (for example, blocks of 4×4, 8×8, 4×8, or 16×16 sampleseach) and coded on a block-by-block basis. Blocks may be codedpredictively with reference to other (already coded) blocks asdetermined by the coding assignment applied to the blocks' respectivepictures. For example, blocks of I pictures may be codednon-predictively or they may be coded predictively with reference toalready coded blocks of the same picture (spatial prediction or intraprediction). Pixel blocks of P pictures may be coded predictively, viaspatial prediction or via temporal prediction with reference to onepreviously coded reference picture. Blocks of B pictures may be codedpredictively, via spatial prediction or via temporal prediction withreference to one or two previously coded reference pictures.

The video encoder (403) may perform coding operations according to apredetermined video coding technology or standard, such as ITU-T Rec.H.265. In its operation, the video encoder (403) may perform variouscompression operations, including predictive coding operations thatexploit temporal and spatial redundancies in the input video sequence.The coded video data, therefore, may conform to a syntax specified bythe video coding technology or standard being used.

In an embodiment, the transmitter (440) may transmit additional datawith the encoded video. The source coder (430) may include such data aspart of the coded video sequence. Additional data may comprisetemporal/spatial/SNR enhancement layers, other forms of redundant datasuch as redundant pictures and slices, SEI messages, VUI parameter setfragments, and so on.

A video may be captured as a plurality of source pictures (videopictures) in a temporal sequence. Intra-picture prediction (oftenabbreviated to intra prediction) makes use of spatial correlation in agiven picture, and inter-picture prediction makes uses of the (temporalor other) correlation between the pictures. In an example, a specificpicture under encoding/decoding, which is referred to as a currentpicture, is partitioned into blocks. When a block in the current pictureis similar to a reference block in a previously coded and still bufferedreference picture in the video, the block in the current picture can becoded by a vector that is referred to as a motion vector. The motionvector points to the reference block in the reference picture, and canhave a third dimension identifying the reference picture, in casemultiple reference pictures are in use.

In some embodiments, a bi-prediction technique can be used in theinter-picture prediction. According to the bi-prediction technique, tworeference pictures, such as a first reference picture and a secondreference picture that are both prior in decoding order to the currentpicture in the video (but may be in the past and future, respectively,in display order) are used. A block in the current picture can be codedby a first motion vector that points to a first reference block in thefirst reference picture, and a second motion vector that points to asecond reference block in the second reference picture. The block can bepredicted by a combination of the first reference block and the secondreference block.

Further, a merge mode technique can be used in the inter-pictureprediction to improve coding efficiency.

According to some embodiments of the disclosure, predictions, such asinter-picture predictions and intra-picture predictions are performed inthe unit of blocks. For example, according to the HEVC standard, apicture in a sequence of video pictures is partitioned into coding treeunits (CTU) for compression, the CTUs in a picture have the same size,such as 64×64 pixels, 32×32 pixels, or 16×16 pixels. In general, a CTUincludes three coding tree blocks (CTBs), which are one luma CTB and twochroma CTBs. Each CTU can be recursively quadtree split into one ormultiple coding units (CUs). For example, a CTU of 64×64 pixels can besplit into one CU of 64×64 pixels, or 4 CUs of 32×32 pixels, or 16 CUsof 16×16 pixels. In an example, each CU is analyzed to determine aprediction type for the CU, such as an inter prediction type or an intraprediction type. The CU is split into one or more prediction units (PUs)depending on the temporal and/or spatial predictability. Generally, eachPU includes a luma prediction block (PB), and two chroma PBs. In anembodiment, a prediction operation in coding (encoding/decoding) isperformed in the unit of a prediction block. Using a luma predictionblock as an example of a prediction block, the prediction block includesa matrix of values (e.g., luma values) for pixels, such as 8×8 pixels,16×16 pixels, 8×16 pixels, 16×8 pixels, and the like.

FIG. 5 shows a diagram of a video encoder (503) according to anotherembodiment of the disclosure. The video encoder (503) is configured toreceive a processing block (e.g., a prediction block) of sample valueswithin a current video picture in a sequence of video pictures, andencode the processing block into a coded picture that is part of a codedvideo sequence. In an example, the video encoder (503) is used in theplace of the video encoder (203) in the FIG. 2 example.

In an HEVC example, the video encoder (503) receives a matrix of samplevalues for a processing block, such as a prediction block of 8×8samples, and the like. The video encoder (503) determines whether theprocessing block is best coded using intra mode, inter mode, orbi-prediction mode using, for example, rate-distortion optimization.When the processing block is to be coded in intra mode, the videoencoder (503) may use an intra prediction technique to encode theprocessing block into the coded picture; and when the processing blockis to be coded in inter mode or bi-prediction mode, the video encoder(503) may use an inter prediction or bi-prediction technique,respectively, to encode the processing block into the coded picture. Incertain video coding technologies, merge mode can be an inter pictureprediction submode where the motion vector is derived from one or moremotion vector predictors without the benefit of a coded motion vectorcomponent outside the predictors. In certain other video codingtechnologies, a motion vector component applicable to the subject blockmay be present. In an example, the video encoder (503) includes othercomponents, such as a mode decision module (not shown) to determine themode of the processing blocks.

In the FIG. 5 example, the video encoder (503) includes the interencoder (530), an intra encoder (522), a residue calculator (523), aswitch (526), a residue encoder (524), a general controller (521), andan entropy encoder (525) coupled together as shown in FIG. 5.

The inter encoder (530) is configured to receive the samples of thecurrent block (e.g., a processing block), compare the block to one ormore reference blocks in reference pictures (e.g., blocks in previouspictures and later pictures), generate inter prediction information(e.g., description of redundant information according to inter encodingtechnique, motion vectors, merge mode information), and calculate interprediction results (e.g., predicted block) based on the inter predictioninformation using any suitable technique. In some examples, thereference pictures are decoded reference pictures that are decoded basedon the encoded video information.

The intra encoder (522) is configured to receive the samples of thecurrent block (e.g., a processing block), in some cases compare theblock to blocks already coded in the same picture, generate quantizedcoefficients after transform, and in some cases also intra predictioninformation (e.g., an intra prediction direction information accordingto one or more intra encoding techniques). In an example, the intraencoder (522) also calculates intra prediction results (e.g., predictedblock) based on the intra prediction information and reference blocks inthe same picture.

The general controller (521) is configured to determine general controldata and control other components of the video encoder (503) based onthe general control data. In an example, the general controller (521)determines the mode of the block, and provides a control signal to theswitch (526) based on the mode. For example, when the mode is the intramode, the general controller (521) controls the switch (526) to selectthe intra mode result for use by the residue calculator (523), andcontrols the entropy encoder (525) to select the intra predictioninformation and include the intra prediction information in thebitstream; and when the mode is the inter mode, the general controller(521) controls the switch (526) to select the inter prediction resultfor use by the residue calculator (523), and controls the entropyencoder (525) to select the inter prediction information and include theinter prediction information in the bitstream.

The residue calculator (523) is configured to calculate a difference(residue data) between the received block and prediction resultsselected from the intra encoder (522) or the inter encoder (530). Theresidue encoder (524) is configured to operate based on the residue datato encode the residue data to generate the transform coefficients. In anexample, the residue encoder (524) is configured to convert the residuedata from a spatial domain to a frequency domain, and generate thetransform coefficients. The transform coefficients are then subject toquantization processing to obtain quantized transform coefficients. Invarious embodiments, the video encoder (503) also includes a residuedecoder (528). The residue decoder (528) is configured to performinverse-transform, and generate the decoded residue data. The decodedresidue data can be suitably used by the intra encoder (522) and theinter encoder (530). For example, the inter encoder (530) can generatedecoded blocks based on the decoded residue data and inter predictioninformation, and the intra encoder (522) can generate decoded blocksbased on the decoded residue data and the intra prediction information.The decoded blocks are suitably processed to generate decoded picturesand the decoded pictures can be buffered in a memory circuit (not shown)and used as reference pictures in some examples.

The entropy encoder (525) is configured to format the bitstream toinclude the encoded block. The entropy encoder (525) is configured toinclude various information according to a suitable standard, such asthe HEVC standard. In an example, the entropy encoder (525) isconfigured to include the general control data, the selected predictioninformation (e.g., intra prediction information or inter predictioninformation), the residue information, and other suitable information inthe bitstream. Note that, according to the disclosed subject matter,when coding a block in the merge submode of either inter mode orbi-prediction mode, there is no residue information.

FIG. 6 shows a diagram of a video decoder (610) according to anotherembodiment of the disclosure. The video decoder (610) is configured toreceive coded pictures that are part of a coded video sequence, anddecode the coded pictures to generate reconstructed pictures. In anexample, the video decoder (610) is used in the place of the videodecoder (210) in the FIG. 2 example.

In the FIG. 6 example, the video decoder (610) includes an entropydecoder (671), an inter decoder (680), a residue decoder (673), areconstruction module (674), and an intra decoder (672) coupled togetheras shown in FIG. 6.

The entropy decoder (671) can be configured to reconstruct, from thecoded picture, certain symbols that represent the syntax elements ofwhich the coded picture is made up. Such symbols can include, forexample, the mode in which a block is coded (such as, for example, intramode, inter mode, bi-predicted mode, the latter two in merge submode oranother submode), prediction information (such as, for example, intraprediction information or inter prediction information) that canidentify certain sample or metadata that is used for prediction by theintra decoder (672) or the inter decoder (680), respectively, residualinformation in the form of, for example, quantized transformcoefficients, and the like. In an example, when the prediction mode isinter or bi-predicted mode, the inter prediction information is providedto the inter decoder (680); and when the prediction type is the intraprediction type, the intra prediction information is provided to theintra decoder (672). The residual information can be subject to inversequantization and is provided to the residue decoder (673).

The inter decoder (680) is configured to receive the inter predictioninformation, and generate inter prediction results based on the interprediction information.

The intra decoder (672) is configured to receive the intra predictioninformation, and generate prediction results based on the intraprediction information.

The residue decoder (673) is configured to perform inverse quantizationto extract de-quantized transform coefficients, and process thede-quantized transform coefficients to convert the residual from thefrequency domain to the spatial domain. The residue decoder (673) mayalso require certain control information (to include the QuantizerParameter (QP)), and that information may be provided by the entropydecoder (671) (data path not depicted as this may be low volume controlinformation only).

The reconstruction module (674) is configured to combine, in the spatialdomain, the residual as output by the residue decoder (673) and theprediction results (as output by the inter or intra prediction modulesas the case may be) to form a reconstructed block, that may be part ofthe reconstructed picture, which in turn may be part of thereconstructed video. It is noted that other suitable operations, such asa deblocking operation and the like, can be performed to improve thevisual quality.

It is noted that the video encoders (203), (403), and (503), and thevideo decoders (210), (310), and (610) can be implemented using anysuitable technique. In an embodiment, the video encoders (203), (403),and (503), and the video decoders (210), (310), and (610) can beimplemented using one or more integrated circuits. In anotherembodiment, the video encoders (203), (403), and (403), and the videodecoders (210), (310), and (610) can be implemented using one or moreprocessors that execute software instructions.

Aspects of the disclosure provide techniques for decoder side motionvector (MV) derivation in hybrid video coding technologies. Morespecifically, in some embodiments, shorter filters for the decoder sidemotion vector derivation (DMVD) are used to reduce the memory bandwidthduring the sample interpolation in DMVD. In addition, in someembodiments, intra prediction constraints are used to reduce the codingdependency of DMVD coded blocks.

Referring to FIG. 7, a current block (701) comprises samples that havebeen found by the encoder during the motion search process to bepredictable from a previous block of the same size that has beenspatially shifted. Instead of coding that MV directly, the MV can bederived from metadata associated with one or more reference pictures,for example from the most recent (in decoding order) reference picture,using the MV associated with either one of five surrounding samples,denoted A0, A1, and B0, B1, B2 (702 through 706, respectively). In someexamples, the MV prediction can use predictors from the same referencepicture that the neighboring block is using.

In some embodiments, a merge mode for inter-picture prediction is used.In an example, when a merge flag (including skip flag) is signaled astrue, a merge index is then signaled to indicate which candidate in amerge candidate list is used to indicate the motion vectors of thecurrent block. At decoder, a merge candidate list is constructed basedon spatial and temporal neighbors of the current block. As shown in FIG.7, neighboring MVs of A0, A1, and B0, B1, B2 can be added into the mergecandidate list. In addition, an MV from temporal neighbors of thecurrent block is added into the merge candidate list in an example. Itis noted that additional merge candidates, such as combinedbi-predictive candidates and zero motion vector candidates, and the likecan be added into the merge candidate list.

According to an aspect of the disclosure, decoder side motion vectorrefinement (DMVR) is one of the DMVD techniques and is used toimprove/refine MV based on starting points.

In some examples, in the case of bi-prediction operation, for theprediction of one block region, two prediction blocks, formedrespectively using an MV0 of a first candidate list list0 and an MV1 ofa second candidate list list1, are combined to form a single predictionsignal that is referred to as a bilateral template. In the DMVR method,the two motion vectors MV0 and MV1 of the bi-prediction are furtherrefined by a bilateral template matching process. The bilateral templatematching applied in the decoder to perform a distortion-based searchbetween the bilateral template and the reconstruction samples in thereference pictures to obtain a refined MV without transmission ofadditional motion information.

FIG. 8 shows an example of DMVR that is based on bilateral templatematching. In DMVR, the bilateral template (840) is generated as theweighted combination (i.e. average) of the two prediction blocks (820)and (830), from the initial MV0 of the first candidate list list0 andMV1 of the second candidate list list1, respectively, as shown in FIG.8. The template matching operation includes calculating cost measuresbetween the generated template (840) and the sample region (around theinitial prediction block) in the reference pictures Ref0 and Ref1. Foreach of the two reference pictures Ref0 and Ref1, the MV that yields theminimum template cost is considered as the updated MV of that list toreplace the original MV. For example, MV0′ replaces MV0, and MV1′replaces MV1. In some examples, nine MV candidates are searched for eachlist. The nine MV candidates include the original MV and 8 surroundingMVs with one luma sample offset to the original MV in either thehorizontal or vertical direction, or both. Finally, the two new MVs,i.e., MV0′ and MV1′ as shown in FIG. 8, are used for generating thefinal bi-prediction results for the current block. A sum of absolutedifferences (SAD) can be used as the cost measure.

In some examples, DMVR is applied for the merge mode of bi-predictionwith one MV from a reference picture in the past and another MV from areference picture in the future, without the transmission of additionalsyntax elements. In an example, DMVR is applied in the merge mode andskip mode, when the condition in (Eq. 1) is true:

(POC−POC0)×(POC−POC1)<0  (Eq. 1)

where POC denotes picture order counter of the current picture, and POC0and POC1 denote picture order counts of the two reference pictures forthe current picture.

In some embodiments, based on signals in the received bitstream, a pairof merge candidates is determined and used as input to DMVR process. Forexample, the pair of merge candidates is denoted as initial motionvectors (MV0, MV1). In some examples, the search points that aresearched by DMVR obey the motion vector difference mirroring condition.In other words, the points that are checked by DMVR, denoted by a pairof candidate motion vectors (MV0′, MV1′), obey (Eq. 2) and (Eq. 3):

MV0′=MV0+MV_(diff)  (Eq. 2)

MV1′=MV1−MV_(diff)  (Eq. 3)

where MV_(diff) denotes the motion vector difference between a candidatemotion vector and an initial motion vector in one of the referencepictures.

FIG. 9 shows a diagram of a first portion (910) of the search space in afirst reference picture and a second search portion (920) of the searchspace in a second reference picture according to an embodiment of thedisclosure. The initial motion vector MV0 points to a point (911) in thefirst portion (910) of the search space, and the initial motion vectorMV1 points to a point (921) in the second portion (920) of the searchspace. Further, the candidate motion vector MV0′ points to a point (912)in the first portion (910) of the search space, and the candidate motionvector MV1′ points to a point (922) in the second portion (920) of thesearch space. The points (912) and (922) satisfy the motion vectordifference mirroring condition. Similarly, points (913) and (923)satisfy the motion vector difference mirroring condition; points (914)and (924) satisfy the motion vector difference mirroring condition;points (915) and (925) satisfy the motion vector difference mirroringcondition; and points (916) and (926) satisfy the motion vectordifference mirroring condition. In the FIG. 9 example, 6 pairs of searchpoints are selected in the search space, and the points (911) and (921)are referred to as center points of the search space.

In some examples, after the construction of the search space, theuni-lateral predictions are respectively performed on the search pointsin the first portion (910) and the second portion (920) of the searchspace using interpolation filters, such as discrete cosine transform(DCTIF) interpolation filter. Further, bilateral matching cost functionis calculated by using mean reduced sum of average difference (MRSAD)between the two uni-lateral predictions for each pair of search points,and then pair of the search points that results of the minimum cost(minimum bilateral matching cost, minimum MRSAD) is selected as therefined MV pair. In an example, for the MRSAD calculation, 16-bitprecision of samples is used (which is the output of the interpolationfiltering), and no clipping and no rounding operations are appliedbefore MRSAD calculation. The reason for not applying rounding andclipping is to reduce internal buffer requirement.

In some embodiments, the integer precision search points are chosenusing an adaptive pattern method. In an example, the cost (bilateralmatching cost) corresponding to the central points (such as (911) and(921) pointed by the initial motion vectors) is calculated firstly. The4 other costs, such as cost corresponding to points (912) and (922),cost corresponding to points (913) and (923), cost corresponding topoints (914) and (924), and cost corresponding to points (915) and(925), are calculated. The distance from the points (912)-(915) to thecenter point (911) is integer number of sample resolution, such as 1pixel (1-pel), and the distance from the points (922)-(925) to thecenter point (921) is also integer number of sample resolution.

Then, based on the result of the 4 other costs, the 6^(th) pair ofsearch points, such as the points (916) and (926) are chosen by thegradient of the previous calculated costs. For example, when the cost ofthe search points (912) and (922) is smaller than the cost of the searchpoints (912) and (923), and the cost of the search points (915) and(925) is smaller than the cost of the search points (914) and (924),then the points (916) and (926) are selected as the 6^(th) pair ofsearch points. In another example, when the cost of the search points(912) and (922) is smaller than the cost of the search points (912) and(923), and the cost of the search points (914) and (924) is smaller thanthe cost of the search points (915) and (925), then the points (917) and(927) are selected as the 6^(th) pair of search points. Then, within the6 pairs of search points, the pair of search points with the minimalcost is used to determine the refined motion vector pair (correspondingto the pair of search points with the minimal cost) that is the outputof one iteration of the DMVR process.

In some embodiments, after one iteration, when the minimum cost isachieved at the central points (e.g., 911 and 921) of the search space,i.e. the motion vectors are not changed, and the refinement process isterminated. Otherwise, the search points with the minimal cost are usedas the new center points to start another iteration of the DMVR process.For example, when the points (916) and (926) have the minimal cost, thenpoints (916) and (926) are used as center points to continue a nextiteration of DMVR process when the search range is not exceeded.

In some examples, when the integer precision search is terminated, halfsample precision search is applied when the application of half-pelsearch does not exceed the search range.

FIG. 10 shows a diagram for half-sample precision search in an example.In the FIG. 10 example, four search points (1020) are distanced to thecenter point (1010) by 1 pixel (1-pel), and can be used as the integerprecision search points (e.g., points 912-915). Further, the four points(1030) are distanced to the center point (1010) by half pixel, and areused in the half-sample precision search. Similarly to the integersample precision search, 4 MRSAD calculations are performed,corresponding to four pair of points with half-pel distance to thecenter points. In an example, the central points in the half-sampleprecision search correspond to the refined motion vector pair that isresulted from the integer precision search with the minimal cost.

Aspects of the disclosure also provide intra prediction constraints toreduce the coding dependency on DMVD coded blocks. In some embodiments,in a picture, some of the blocks are coded in the inter prediction modesand some of the blocks are coded in the intra prediction modes.

In intra prediction, sample values are represented without reference tosamples or other data from previously reconstructed reference pictures.In some examples, a predictor block can be formed using neighboringsample values belonging to already available samples. Sample values ofneighboring samples are copied into the predictor block according to adirection. A reference to the direction in use can be coded in thebitstream or may itself be predicted. For intra prediction, intraprediction modes can be pre-defined corresponding to the directions.

FIG. 11 shows a schematic illustration of exemplary intra predictionmodes. In the FIG. 1 example, depicted in the lower right is a subset ofnine predictor directions known from H.265's 33 possible predictordirections (corresponding to the 33 angular modes of the 35 intraprediction modes). The point where the arrows converge (1101) representsthe sample being predicted. The arrows represent the direction fromwhich the sample is being predicted. For example, arrow (1102) indicatesthat sample (1101) is predicted from a sample or samples to the upperright, at a 45 degree angle from the horizontal. Similarly, arrow (1103)indicates that sample (1101) is predicted from a sample or samples tothe lower left of sample (1101), in a 22.5 degree angle from thehorizontal.

Still referring to FIG. 11, on the top left there is depicted a squareblock (1104) of 4×4 samples (indicated by a dashed, boldface line). Thesquare block (1104) includes 16 samples, each labelled with an “S”, itsposition in the Y dimension (e.g., row index) and its position in the Xdimension (e.g., column index). For example, sample S21 is the secondsample in the Y dimension (from the top) and the first (from the left)sample in the X dimension. Similarly, sample S44 is the fourth sample inblock (1104) in both the Y and X dimensions. As the block is 4×4 samplesin size, S44 is at the bottom right. Further shown are reference samplesthat follow a similar numbering scheme. A reference sample is labelledwith an R, its Y position (e.g., row index) and X position (columnindex) relative to block (1104). In both H.264 and H.265, predictionsamples neighbor the block under reconstruction; therefore no negativevalues need to be used.

Intra picture prediction (also referred to as intra prediction) can workby copying reference sample values from the neighboring samples asappropriated by the signaled prediction direction. For example, assumethe coded video bitstream includes signaling that, for this block,indicates a prediction direction consistent with arrow (1102)—that is,samples are predicted from a prediction sample or samples to the upperright, at a 45 degree angle from the horizontal. In that case, samplesS41, S32, S23, and S14 are predicted from the same reference sample R05.Sample S44 is then predicted from reference sample R08.

In certain cases, the values of multiple reference samples may becombined, for example through interpolation using interpolation filter,in order to calculate a reference sample; especially when the directionsare not evenly divisible by 45 degrees.

According to an aspect of the disclosure, a flag called“constrained_intra_pred_flag” is signaled (in the video bitstream) inthe Picture Parameter Set (PPS) to indicate whether constrained intraprediction is applied. When this flag is equal to 0, the referencesamples used in the intra prediction may only come from reconstructedintra blocks (blocks that are reconstructed using intra predictionmodes). When this flag is equal to 1, they may come from reconstructedblocks coded in either intra or inter modes.

In some examples, the filters for interpolation in motion search andfinal motion compensation in DMVR are proposed to reduce the memorybandwidth required to load the reference samples. In some examples, aunified filter is used for the final motion compensation in both DMVDand non-DMVD mode under certain conditions. In some examples, a latencyregion is proposed, from which only the non-refined MV could be used asa spatial MV predictor.

Aspects of the disclosure provide methods to improve performance ofvideo coding that uses DMVD. In certain implementations, whenconstrained intra prediction is disabled, the intra prediction dependson reconstruction of, for example, a block based on DMVD. The DMVD takesrelatively long time and increases the coding latency. Also, thecondition for using a unified DMVD motion compensation filter is toostrict and may degrade the coding performance.

The proposed methods may be used separately or combined in any order.Further, the methods (or embodiments) may be implemented by processingcircuitry (e.g., one or more processors or one or more integratedcircuits). In one example, the one or more processors execute a programthat is stored in a non-transitory computer-readable medium.

In the following, the term block may be interpreted as a predictionblock (or a prediction unit, i.e. PU), a coding block, or a coding unit,i.e. CU.

According to an aspect of the disclosure, the constrains for intraprediction are extended to take into account the DMVD process.Specifically, in some examples, in addition to the options in HEVC wherethe reconstructed samples used in intra prediction may be intra-only orboth intra and inter coded, the DMVD mode is constrained and excludedfrom the reconstructed sample. For example, when a block isreconstructed according to a DMVD process, a specific flag that isassociated with the block is turned on to indicate that the block isreconstructed according to the DMVD process, and the block is excludedfrom being a reference block for another block to be reconstructed in anintra prediction mode.

According to an aspect of the disclosure, the flag that is referred toas the constrained_intra_pred_flag in the PPS can have multiple valuesto indicate different options. In an embodiment, theconstrained_intra_pred_flag can be 0, 1, or 2. When theconstrained_intra_pred_flag is equal to 0, the reference samples used inan intra prediction mode may only come from reconstructed intra blocks(blocks that are reconstructed using intra prediction modes). When theconstrained_intra_pred_flag is equal to 1, the reference samples used inan intra prediction mode may come from reconstructed blocks coded ineither intra prediction modes or inter prediction modes. When theconstrained_intra_pred_flag is not equal to 0 and is not equal to 1, forexample, the constrained_intra_pred_flag is equal to 2, the referencesamples used in an intra prediction mode may only be derived from ablock that is not reconstructed by DMVD process (e.g., the specific flagof DMVD for the block is not turned on).

According to another aspect of the disclosure, another flag, which isreferred to as constrained_intra_pred_dmvd_flag, may be signaled (in thevideo bitstream) to indicate whether DMVD reconstructed samples arerestricted from intra prediction modes. In an example, when theconstrained_intra_pred_dmvd_flag is equal to 0, the constrained intraprediction is applied in the same way as in HEVC, for example based onthe constrained_intra_pred_flag. When the constrained_intra_pred_flag isequal to 0, the reference samples used in an intra prediction mode mayonly come from reconstructed intra blocks (blocks that are reconstructedusing intra prediction modes). When the constrained_intra_pred_flag isequal to 1, the reference samples used in an intra prediction mode maycome from reconstructed blocks coded in either intra prediction modes orinter prediction modes.

In another example, when the constrained intrapred dmvd flag is equal to1, during reconstruction, blocks in the DMVD modes are excluded frombeing reference blocks in the intra prediction modes, and the referencesamples used in an intra prediction mode may only be derived from ablock that is not reconstructed by DMVD process (e.g., the specific flagof DMVD for the block is not turned on).

In an embodiment, when the constrained_intra_pred_flag is equal to 0,the flag constrained intrapred dmvd flag is explicitly signaled, forexample, immediately after the constrained_intra_pred_flag in the videobitstream. At the decoder side, the decoder can extract the constrainedintrapred dmvd flag from the video bitstream.

In another embodiment, when the constrained_intra_pred_flag is equal to0, the constrained intrapred dmvd flag is not signaled and is inferredto be 1. At the decoder side, the decoder infers that the constrainedintrapred dmvd flag is 1 when the constrained_intra_pred_flag is equalto 0.

In another embodiment, the intra reference samples are allowed to comefrom a DMVD coded block when the DMVD coded block is inside the DMVRnon-latency regions of the current block.

FIG. 12 shows a diagram illustrating DMVR latency region and DMVRnon-latency region according to an embodiment of the disclosure. In theFIG. 12 example, block 4 is the current block that is underreconstruction using intra prediction. The DMVR non-latency regionrefers to a region that can be fully reconstructed using DMVR processbefore the reconstruction of the current block. The latency regionrefers to a region that is not able to be fully reconstructed using DMVRprocess before the reconstruction of the current block. For example,block 1 is in the DMVR non-latency region, and other blocks that arereconstructed before the block 1, such as block 0, are also in the DMVRnon-latency region. Further, block 2 is in DMVR latency region, andother blocks that are reconstructed after the block 2 and before thecurrent block 4, such as block 3, are also in the DMVR latency region.In some examples, regardless of the constrained_intra_pred_flag orconstrained intrapred dmvd flag, a decoder can use reconstructed samplesin block 1 for intra prediction in block 4 according to intraprediction.

According to some aspects of the disclosure, in DMVR, differentinterpolation filters can be used at different steps in DMVR process. Insome embodiments, a first filter of M-tap, referred to as a first filterf_(M), is used in the motion search portion (to refine the MVs) of theDMVR process and a second filter of N-tap, referred to as a secondfilter f_(N), is used in the final motion compensation (after therefined MVs are finalized) of the DMVR. M and N are two integer numbers(such as 2, 4, 6, or 8), and may or may not be equal. In someembodiments, a third filter of L-tap, referred to as a third filterf_(L), is used in the interpolation for non-DMVD modes. M, N, and L arepre-determined for each block and not signaled in the video bitstream.In VVC, L is equal to 8 in an example.

In some embodiments, M is constrained to be smaller than or equal to N.For example, M=4, N=4, or M=4, N=8. Further, N is constrained to besmaller than or equal to L. In an embodiment, M and N are not fixednumbers, and regardless how M and N change, M is constrained to be lessthan or equal to N.

In some embodiments, to reduce the memory bandwidth in DMVD, certainrestrictions are proposed on the first filter f_(M) for the motionsearch and the second filter f_(N) for the final motion compensation. Insome examples, the constrains are represented by Eq. 4:

2×SR+M≤L  (Eq. 4)

where SR denotes the search range (in pixel) in DMVD. For example, whenL is equal to 8, SR is equal to 1, then M is smaller than or equal tosix. When L is equal to 8, SR is equal to 2, then M is smaller than orequal to four.

During the DMVR process in some embodiments, after the refined MV hasbeen derived, when the corresponding integer parts of the refined MV andthe initial MV are equal, then the third filter f_(L) is used as theinterpolation filter for the final motion compensation, and N is set toequal L. Otherwise, when the corresponding integer parts of the refinedMV and the initial MV are not equal, the first filter f_(M) is the usedas the interpolation filter for the final motion compensation, and N isset to equal M.

In some embodiments, when M or N is equal to 4, the first filter f_(M)and/or the second filter f_(N) can be selected to have lower cutofffrequency than the third filter f_(L) used in non-DMVD modes is used.Table 1 shows an example of a 4-tap filter that can be used as the firstfilter f_(M) and/or the second filter f_(N). Table 1 includescoefficients corresponding to phases. Thus, when an interpolation phaseis determined, the coefficients of the 4-tap filter for theinterpolation phase can be quickly determined based on a lookup in theTable 1.

TABLE 1 Coefficients (multiplied by 64) for a 4-tap filter f₄ phasecoefficients 0/16 0, 64, 0, 0 1/16 −2, 63, 4, −1 2/16 −4, 62, 8, −2 3/16−5, 59, 13, −3 4/16 −6, 56, 18, −4 5/16 −6, 52, 23, −5 6/16 −7, 48, 28,−5 7/16 −7, 44, 33, −6 8/16 −7, 39, 39, −7 9/16 −6, 33, 44, −7 10/16 −5, 28, 48, −7 11/16  −5, 23, 52, −6 12/16  −4, 18, 56, −6 13/16  −3,13, 59, −5 14/16  −2, 8, 62, −4 15/16  −1, 4, 63, −2

In some embodiments, when M or N is equal to 6, the first filter f_(M)and/or the second filter f_(N) can be selected to have lower cutofffrequency than the third filter f_(L) used in non-DMVD modes is used.Table 2 shows an example of a 6-tap filter that can be used as the firstfilter f_(M) and/or the second filter f_(N). Table 2 includescoefficients corresponding to phases. Thus, when an interpolation phaseis determined, the coefficients of the 6-tap filter for theinterpolation phase can be quickly determined based on a lookup in theTable 2.

TABLE 2 Coefficients (multiplied by 64) for a 6-tap filter f₆ phasecoefficients 0/16 0, 0, 64, 0, 0, 0 1/16 1, −3, 64, 4, −2, 0 2/16 1, −6,62, 9, −3, 1 3/16 2, −8, 60, 14, −5, 1 4/16 2, −9, 57, 19, −7, 2 5/16 3,−10, 53, 24, −8, 2 6/16 3, −11, 50, 29, −9, 2 7/16 3, −11, 44, 35, −10,3 8/16 1, −7, 38, 38, −7, 1 9/16 3, −10, 35, 44, −11, 3 10/16  2, −9,29, 50, −11, 3 11/16  2, −8, 24, 53, −10, 3 12/16  2, −7, 19, 57, −9, 213/16  1, −5, 14, 60, −8, 2 14/16  1, −3, 9, 62, −6, 1 15/16  0, −2, 4,64, −3, 1

FIG. 13 shows a flow chart outlining a process (1300) according to anembodiment of the disclosure. The process (1300) can be used in thereconstruction of a block coded in intra mode, so to generate aprediction block for the block under reconstruction. In variousembodiments, the process (1300) are executed by processing circuitry,such as the processing circuitry in the terminal devices (110), (120),(130) and (140), the processing circuitry that performs functions of thevideo encoder (203), the processing circuitry that performs functions ofthe video decoder (210), the processing circuitry that performsfunctions of the video decoder (310), the processing circuitry thatperforms functions of the video encoder (403), and the like. In someembodiments, the process (1300) is implemented in software instructions,thus when the processing circuitry executes the software instructions,the processing circuitry performs the process (1300). The process startsat (S1301) and proceeds to (S1310).

At (S1310), a constrain flag is decoded. The constrain flag isindicative of an exclusion of DMVD for reference sample reconstruction.In an example, a constrain flag, such as constrained_intra_flag, is ahigh-level flag that is decoded from the Picture Parameter Set (PPS)from the coded video bitstream.

At (S1320), prediction information of a current block is decoded fromthe coded video bitstream. The prediction information is indicative ofan intra prediction mode.

At (S1330), reference samples for reconstructing a sample of the currentblock is determined. The reference samples are in a same picture as thecurrent block and are determined based on the intra prediction mode andbased on the exclusion of the DMVD. In some examples, the referencesamples are reconstructed without using the DMVD. In another example,the reference samples are within a DMVD non-latency region with regardto the current block.

At (S1340), the sample of the current block is reconstructed based onthe reference samples. Then the process proceeds to (S1399) andterminates.

FIG. 14 shows a flow chart outlining a process (1400) according to anembodiment of the disclosure. The process (1400) can be used in thereconstruction of a block coded in intra mode, so to generate aprediction block for the block under reconstruction. In variousembodiments, the process (1400) are executed by processing circuitry,such as the processing circuitry in the terminal devices (110), (120),(130) and (140), the processing circuitry that performs functions of thevideo encoder (203), the processing circuitry that performs functions ofthe video decoder (210), the processing circuitry that performsfunctions of the video decoder (310), the processing circuitry thatperforms functions of the video encoder (403), and the like. In someembodiments, the process (1400) is implemented in software instructions,thus when the processing circuitry executes the software instructions,the processing circuitry performs the process (1400). The process startsat (S1401) and proceeds to (S1410).

At (S1410), prediction information of a current block is decoded fromthe coded video bitstream. The prediction information indicates an interprediction mode that includes a motion vector refining step and a finalmotion compensation step.

At (S1420), a first interpolation filter is used during the motionvector refining step to determine a first refined motion vector and asecond refined motion vector. The first refined motion vector isindicative of a first refined reference block in a first referenceframe. The second refined motion vector is indicative of a secondrefined reference block in a second reference frame.

At (S1430), a second interpolation filter that is different from thefirst interpolation filter is used during the final motion compensationstep to reconstruct the current block according to the first refinedmotion vector and the second refined motion vector. Then the processproceeds to (S1499) and terminate.

The techniques described above, can be implemented as computer softwareusing computer-readable instructions and physically stored in one ormore computer-readable media. For example, FIG. 15 shows a computersystem (1500) suitable for implementing certain embodiments of thedisclosed subject matter.

The computer software can be coded using any suitable machine code orcomputer language, that may be subject to assembly, compilation,linking, or like mechanisms to create code comprising instructions thatcan be executed directly, or through interpretation, micro-codeexecution, and the like, by one or more computer central processingunits (CPUs), Graphics Processing Units (GPUs), and the like.

The instructions can be executed on various types of computers orcomponents thereof, including, for example, personal computers, tabletcomputers, servers, smartphones, gaming devices, internet of thingsdevices, and the like.

The components shown in FIG. 15 for computer system (1500) are exemplaryin nature and are not intended to suggest any limitation as to the scopeof use or functionality of the computer software implementingembodiments of the present disclosure. Neither should the configurationof components be interpreted as having any dependency or requirementrelating to any one or combination of components illustrated in theexemplary embodiment of a computer system (1500).

Computer system (1500) may include certain human interface inputdevices. Such a human interface input device may be responsive to inputby one or more human users through, for example, tactile input (such as:keystrokes, swipes, data glove movements), audio input (such as: voice,clapping), visual input (such as: gestures), olfactory input (notdepicted). The human interface devices can also be used to capturecertain media not necessarily directly related to conscious input by ahuman, such as audio (such as: speech, music, ambient sound), images(such as: scanned images, photographic images obtain from a still imagecamera), video (such as two-dimensional video, three-dimensional videoincluding stereoscopic video).

Input human interface devices may include one or more of (only one ofeach depicted): keyboard (1501), mouse (1502), trackpad (1503), touchscreen (1510), data-glove (not shown), joystick (1505), microphone(1506), scanner (1507), camera (1508).

Computer system (1500) may also include certain human interface outputdevices. Such human interface output devices may be stimulating thesenses of one or more human users through, for example, tactile output,sound, light, and smell/taste. Such human interface output devices mayinclude tactile output devices (for example tactile feedback by thetouch-screen (1510), data-glove (not shown), or joystick (1505), butthere can also be tactile feedback devices that do not serve as inputdevices), audio output devices (such as: speakers (1509), headphones(not depicted)), visual output devices (such as screens (1510) toinclude CRT screens, LCD screens, plasma screens, OLED screens, eachwith or without touch-screen input capability, each with or withouttactile feedback capability—some of which may be capable to output twodimensional visual output or more than three dimensional output throughmeans such as stereographic output; virtual-reality glasses (notdepicted), holographic displays and smoke tanks (not depicted)), andprinters (not depicted).

Computer system (1500) can also include human accessible storage devicesand their associated media such as optical media including CD/DVD ROM/RW(1520) with CD/DVD or the like media (1521), thumb-drive (1522),removable hard drive or solid state drive (1523), legacy magnetic mediasuch as tape and floppy disc (not depicted), specialized ROM/ASIC/PLDbased devices such as security dongles (not depicted), and the like.

Those skilled in the art should also understand that term “computerreadable media” as used in connection with the presently disclosedsubject matter does not encompass transmission media, carrier waves, orother transitory signals.

Computer system (1500) can also include an interface to one or morecommunication networks. Networks can for example be wireless, wireline,optical. Networks can further be local, wide-area, metropolitan,vehicular and industrial, real-time, delay-tolerant, and so on. Examplesof networks include local area networks such as Ethernet, wireless LANs,cellular networks to include GSM, 3G, 4G, 5G, LTE and the like, TVwireline or wireless wide area digital networks to include cable TV,satellite TV, and terrestrial broadcast TV, vehicular and industrial toinclude CANBus, and so forth. Certain networks commonly require externalnetwork interface adapters that attached to certain general purpose dataports or peripheral buses (1549) (such as, for example USB ports of thecomputer system (1500)); others are commonly integrated into the core ofthe computer system (1500) by attachment to a system bus as describedbelow (for example Ethernet interface into a PC computer system orcellular network interface into a smartphone computer system). Using anyof these networks, computer system (1500) can communicate with otherentities. Such communication can be uni-directional, receive only (forexample, broadcast TV), uni-directional send-only (for example CANbus tocertain CANbus devices), or bi-directional, for example to othercomputer systems using local or wide area digital networks. Certainprotocols and protocol stacks can be used on each of those networks andnetwork interfaces as described above.

Aforementioned human interface devices, human-accessible storagedevices, and network interfaces can be attached to a core (1540) of thecomputer system (1500).

The core (1540) can include one or more Central Processing Units (CPU)(1541), Graphics Processing Units (GPU) (1542), specialized programmableprocessing units in the form of Field Programmable Gate Areas (FPGA)(1543), hardware accelerators for certain tasks (1544), and so forth.These devices, along with Read-only memory (ROM) (1545), Random-accessmemory (1546), internal mass storage such as internal non-useraccessible hard drives, SSDs, and the like (1547), may be connectedthrough a system bus (1548). In some computer systems, the system bus(1548) can be accessible in the form of one or more physical plugs toenable extensions by additional CPUs, GPU, and the like. The peripheraldevices can be attached either directly to the core's system bus (1548),or through a peripheral bus (1549). Architectures for a peripheral businclude PCI, USB, and the like.

CPUs (1541), GPUs (1542), FPGAs (1543), and accelerators (1544) canexecute certain instructions that, in combination, can make up theaforementioned computer code. That computer code can be stored in ROM(1545) or RAM (1546). Transitional data can be also be stored in RAM(1546), whereas permanent data can be stored for example, in theinternal mass storage (1547). Fast storage and retrieve to any of thememory devices can be enabled through the use of cache memory, that canbe closely associated with one or more CPU (1541), GPU (1542), massstorage (1547), ROM (1545), RAM (1546), and the like.

The computer readable media can have computer code thereon forperforming various computer-implemented operations. The media andcomputer code can be those specially designed and constructed for thepurposes of the present disclosure, or they can be of the kind wellknown and available to those having skill in the computer software arts.

As an example and not by way of limitation, the computer system havingarchitecture (1500), and specifically the core (1540) can providefunctionality as a result of processor(s) (including CPUs, GPUs, FPGA,accelerators, and the like) executing software embodied in one or moretangible, computer-readable media. Such computer-readable media can bemedia associated with user-accessible mass storage as introduced above,as well as certain storage of the core (1540) that are of non-transitorynature, such as core-internal mass storage (1547) or ROM (1545). Thesoftware implementing various embodiments of the present disclosure canbe stored in such devices and executed by core (1540). Acomputer-readable medium can include one or more memory devices orchips, according to particular needs. The software can cause the core(1540) and specifically the processors therein (including CPU, GPU,FPGA, and the like) to execute particular processes or particular partsof particular processes described herein, including defining datastructures stored in RAM (1546) and modifying such data structuresaccording to the processes defined by the software. In addition or as analternative, the computer system can provide functionality as a resultof logic hardwired or otherwise embodied in a circuit (for example:accelerator (1544)), which can operate in place of or together withsoftware to execute particular processes or particular parts ofparticular processes described herein. Reference to software canencompass logic, and vice versa, where appropriate. Reference to acomputer-readable media can encompass a circuit (such as an integratedcircuit (IC)) storing software for execution, a circuit embodying logicfor execution, or both, where appropriate. The present disclosureencompasses any suitable combination of hardware and software.

Appendix A: Acronyms

JEM: joint exploration modelVVC: versatile video codingBMS: benchmark set

MV: Motion Vector HEVC: High Efficiency Video Coding SEI: SupplementaryEnhancement Information VUI: Video Usability Information GOPs: Groups ofPictures TUs: Transform Units, PUs: Prediction Units CTUs: Coding TreeUnits CTBs: Coding Tree Blocks PBs: Prediction Blocks HRD: HypotheticalReference Decoder SNR: Signal Noise Ratio CPUs: Central Processing UnitsGPUs: Graphics Processing Units CRT: Cathode Ray Tube LCD:Liquid-Crystal Display OLED: Organic Light-Emitting Diode CD: CompactDisc DVD: Digital Video Disc ROM: Read-Only Memory RAM: Random AccessMemory ASIC: Application-Specific Integrated Circuit PLD: ProgrammableLogic Device LAN: Local Area Network

GSM: Global System for Mobile communications

LTE: Long-Term Evolution CANBus: Controller Area Network Bus USB:Universal Serial Bus PCI: Peripheral Component Interconnect FPGA: FieldProgrammable Gate Areas

SSD: solid-state drive

IC: Integrated Circuit CU: Coding Unit

While this disclosure has described several exemplary embodiments, thereare alterations, permutations, and various substitute equivalents, whichfall within the scope of the disclosure. It will thus be appreciatedthat those skilled in the art will be able to devise numerous systemsand methods which, although not explicitly shown or described herein,embody the principles of the disclosure and are thus within the spiritand scope thereof.

What is claimed is:
 1. A method for video decoding, comprising: decodinga constrain flag from a coded video bitstream, the constrain flag beingindicative of an exclusion of decoder-side motion vector derivation(DMVD) for reference sample reconstruction; decoding predictioninformation of a current block from the coded video bitstream, theprediction information being indicative of an intra prediction mode;determining, in a same picture as the current block, reference samplesfor a sample in the current block based on the intra prediction mode andbased on the exclusion of the DMVD; and reconstructing the sample of thecurrent block according to the reference samples.
 2. The method of claim1, further comprising: determining the reference samples according tothe intra prediction mode with the reference samples being reconstructedwithout using the DMVD.
 3. The method of claim 1, wherein the constrainflag has a first potential value indicative of a permit of referencesample reconstruction using intra prediction, has a second potentialvalue indicative of a permit of reference sample reconstruction usingintra prediction and inter prediction, and has a third potential valueindicative of the exclusion of the DMVD.
 4. The method of claim 1,further comprising: decoding a first constrain flag indicative of apermit of reference sample reconstruction using intra prediction andinter prediction; and decoding the constrain flag that is a secondconstrain flag indicative of the exclusion of the DMVD for the referencesample construction.
 5. The method of claim 1, further comprising:decoding a first constrain flag indicative of a permit of referencesample reconstruction using intra prediction; and decoding the constrainflag that is a second constrain flag indicative of the exclusion of theDMVD for the reference sample construction.
 6. The method of claim 1,further comprising: decoding a first constrain flag indicative of apermit of reference sample reconstruction using intra prediction; andinferring, based on the first constrain flag, the exclusion of the DMVDfor the reference sample construction.
 7. The method of claim 1, furthercomprising: determining the reference samples in a DMVD non-latencyregion according to the intra prediction mode with the reference samplesavailable for the reconstruction of the current block without delay. 8.A method for video decoding, comprising: decoding prediction informationfor a current block in a current picture from a coded video bitstream,the prediction information being indicative of an inter prediction modethat includes a motion vector refining step and a final motioncompensation step; using a first interpolation filter during the motionvector refining step to determine a first refined motion vector that isindicative of a first refined reference block in a first reference framefrom an initial motion vector; and using a second interpolation filterthat is different from the first interpolation filter during the finalmotion compensation step to reconstruct the current block according tothe first refined motion vector.
 9. The method of claim 8, furthercomprising: using the first interpolation filter during the motionvector refining step to determine the first refined motion vector thatis indicative of a first refined reference block in a first referenceframe and a second refined motion vector that is indicative of a secondrefined reference block in a second reference frame; and using thesecond interpolation filter during the final motion compensation step toreconstruct the current block according to the first refined motionvector and the second refined motion vector.
 10. The method of claim 8,wherein the first interpolation filter has equal or fewer number of tapsthan the second interpolation filter.
 11. The method of claim 8, whereinthe inter prediction mode is decoder-side motion vector derivation(DMVD) mode, and the first interpolation filter and the secondinterpolation filter are different from a third interpolation filterthat is used in a non-DMVD mode.
 12. The method of claim 11, wherein thesecond interpolation filter has equal or fewer number of taps than thethird interpolation filter.
 13. The method of claim 12, wherein asubtraction of a third number of taps of the third interpolation filterand a second number of taps of the second interpolation filter is largerthan two times of the search range in pixel.
 14. The method of claim 11,wherein the first interpolation filter and the second interpolationfilter have lower cutoff frequency than the third interpolation filter.15. The method of claim 11, further comprising: selecting the secondinterpolation filter based on a change in an integer portion of thefirst refined motion vector.
 16. The method of claim 15, furthercomprising: using the third interpolation filter to replace the secondinterpolation filter during the final motion compensation step toreconstruct the current block according to the first refined motionvector when the first refined motion vector and the initial motionvector have the same integer portion; and using the first interpolationfilter to replace the second interpolation filter during the finalmotion compensation step to reconstruct the current block according tothe first refined motion vector when the first refined motion vector andthe initial motion vector have different integer portions.
 17. Anapparatus for video decoding, comprising: processing circuitryconfigured to: decode a constrain flag from a coded video bitstream, theconstrain flag being indicative of an exclusion of decoder-side motionvector derivation (DMVD) for reference sample reconstruction; decodeprediction information of a current block from the coded videobitstream, the prediction information being indicative of an intraprediction mode; determine, in a same picture as the current block,reference samples for a sample in the current block based on the intraprediction mode and based on the exclusion of the DMVD; and reconstructthe sample of the current block according to the reference samples. 18.The apparatus of claim 17, wherein the processing circuitry furtherconfigured to: determine the reference samples according to the intraprediction mode with the reference samples being reconstructed withoutusing the DMVD.
 19. The apparatus of claim 17, wherein the constrainflag has a first potential value indicative of a permit of referencesample reconstruction using intra prediction, has a second potentialvalue indicative of a permit of reference sample reconstruction usingintra prediction and inter prediction, and has a third potential valueindicative of the exclusion of the DMVD.
 20. The apparatus of claim 17,wherein the processing circuitry further configured to: decode a firstconstrain flag indicative of a permit of reference sample reconstructionusing intra prediction and inter prediction; and decode the constrainflag that is a second constrain flag indicative of the exclusion of theDMVD for the reference sample construction.